Article Local Auxin Biosynthesis Is a Key Regulator of Plant Development Graphical Abstract Authors Javier Brumos, Linda M. Robles, Jeonga Yun, Thien C. Vu, Savannah Jackson, Jose M. Alonso, Anna N. Stepanova Correspondence [email protected] In Brief The plant hormone auxin is a key regulator of plant growth and development. Brumos et al. show that local biosynthesis and polar transport of auxin act in concert to produce robust auxin maxima in the root quiescent center to sustain the meristematic potential of the root stem cell niche. Highlights d Local auxin production in roots is required for maintaining functional root meristems d Local biosynthesis and transport of auxin cooperate at generating robust auxin maxima d Auxin produced in the root quiescent center is sufficient for root meristem viability Brumos et al., 2018, Developmental Cell 47, 306–318 November 5, 2018 ª 2018 Elsevier Inc. https://doi.org/10.1016/j.devcel.2018.09.022 Developmental Cell Article Local Auxin Biosynthesis Is a Key Regulator of Plant Development Javier Brumos,1 Linda M. Robles,1 Jeonga Yun,1 Thien C. Vu,1 Savannah Jackson,1 Jose M. Alonso,1 and Anna N. Stepanova1,2,* 1Department of Plant and Microbial Biology, Program in Genetics, North Carolina State University, Raleigh, NC 27695-7614, USA 2Lead Contact *Correspondence: [email protected] https://doi.org/10.1016/j.devcel.2018.09.022 SUMMARY 20 years that a combination of genetic, biochemical, and phar- macological approaches (Zhao et al., 2001; Cheng et al., 2006; Auxin is a major phytohormone that controls Cheng et al., 2007; Stepanova et al., 2008; Tao et al., 2008; numerous aspects of plant development and coordi- Mashiguchi et al., 2011; Stepanova et al., 2011), together nates plant responses to the environment. Morpho- with the development of more sensitive methodologies for genic gradients of auxin govern cell fate decisions auxin metabolite quantification (Nova´ k et al., 2012), have led and underlie plant phenotypic plasticity. Polar auxin to the identification of the first complete pathway of auxin transport plays a central role in auxin maxima gener- biosynthesis in plants. The best characterized auxin in plants, indole-3-acetic acid (IAA), is predominantly produced from ation. The discovery of the exquisite spatiotemporal the aromatic amino acid L-tryptophan (Trp) via indole-3-pyruvic expression patterns of auxin biosynthesis genes of acid (IPyA) in a two-step pathway (reviewed in Brumos et al., WEI8 TAR YUC the / and families suggested that local 2014). Trp is first converted to IPyA by a small family of trypto- auxin production may contribute to the formation of phan aminotransferases that in Arabidopsis is represented by auxin maxima. Herein, we systematically addressed TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS1 the role of local auxin biosynthesis in plant develop- (TAA1) (also known as WEAK ETHYLENE INSENSITIVE8 ment and responses to the stress phytohormone [WEI8] [Stepanova et al., 2008], SHADE AVOIDANCE3 [SAV3] ethylene by manipulating spatiotemporal patterns [Tao et al., 2008], TRANSPORT INHIBITOR RESPONSE2 of WEI8. Our study revealed that local auxin biosyn- [TIR2] [Yamada et al., 2009], and CYTOKININ INDUCED thesis and transport act synergistically and are indi- ROOT CURLING1 [CKRC1] [Zhou et al., 2011]) and its paralogs vidually dispensable for root meristem maintenance. TRYPTOPHAN AMINOTRANSFERASE RELATED1 (TAR1) and TAR2 (Stepanova et al., 2008). IPyA is then converted into In contrast, flower fertility and root responses to IAA by the YUCCA (YUC) family of flavin monooxygenases ethylene require local auxin production that cannot that in Arabidopsis consists of 11 members, YUC1 through be fully compensated for by transport in the genera- YUC11 (reviewed in Zhao, 2014). tion of morphogenic auxin maxima. According to the classical view in the auxin field, a majority of auxin in plants is produced in the shoot apical meristems, young leaves, and flower buds and is rapidly distributed INTRODUCTION throughout the plant via the phloem (reviewed in Teale et al., 2006). In addition, auxin can move more slowly cell to cell via Nearly every aspect of a plant’s life is influenced by auxin. Not polar auxin transport with the help of auxin influx carriers only does this phytohormone govern many developmental pro- AUXIN1 (AUX1)/LIKE-AUXs (LAXs) and auxin efflux transporters grams of the plant but also is able to tune plant growth and PIN FORMED (PINs) and ABCB/MULTIDRUG RESISTANCE development according to the ever-changing external condi- (MDR)/PHOSPHOGLYCOPROTEIN (PGPs) (reviewed in Ada- tions surrounding the plant. In the meristems, auxin regulates mowski and Friml, 2015). It is the polar distribution of PINs cell division, elongation, and differentiation leading to down- within cells that is thought to enable directional flow of auxin stream organogenesis that shapes shoot and root architecture. to generate robust morphogenic auxin gradients that instruct The vital roles of auxin gradients in plant growth and develop- plant development. The classical view has however been chal- ment have been established and interpreted mainly as the lenged by the finding that auxin can also be synthesized locally product of the combined action of auxin transport, signaling in roots (Ljung et al., 2002; Ljung et al., 2005; Stepanova et al., and response. Conversely, little is known about the contribution 2008). Cloning of the auxin biosynthesis genes WEI8/TARs and of the de novo auxin biosynthesis to the generation and main- YUCs uncovered that they are expressed not only in shoots but tenance of the morphogenic auxin maxima. A thorough under- also in roots, and the domains of expression of several of these standing of how and where auxin is produced is nonetheless genes coincide with the auxin response maxima in root tips critical to our ability to precisely define auxin sources and sinks (Stepanova et al., 2008; Chen et al., 2014), as defined by the and thereby to establish a more refined picture of the polar auxin activity reporters DR5:GFP or DII-VENUS (Ulmasov transport system and auxin activity. It is only in the past et al., 1997; Brunoud et al., 2012; Liao et al., 2015). Multiple 306 Developmental Cell 47, 306–318, November 5, 2018 ª 2018 Elsevier Inc. higher order wei8/tar and yuc mutants show dramatic develop- ethylene and provide a convenient platform for elucidating the mental effects (Stepanova et al., 2008; Tao et al., 2008; Cheng respective roles of auxin biosynthesis and transport in a physio- et al., 2007; Cheng et al., 2006), suggesting that local activity of logically relevant context. the biosynthesis genes may have a direct role in the generation Herein, we evaluated the contribution of localized auxin pro- and/or maintenance of auxin maxima. Supporting the key role duction versus polar transport to specific growth and develop- of local biosynthesis is the finding that auxin produced in the ment programs in Arabidopsis. Our data indicate that root-pro- aerial parts of plants is unable to compensate for an auxin duced local sources of auxin are required for root stem cell biosynthesis deficiency of yucQ mutants in roots, resulting in niche maintenance, whereas shoot-derived auxin alone cannot the degeneration of root meristems (Chen et al., 2014). keep the root meristems alive. Local auxin biosynthesis and The discovery of an essential contribution of locally made auxin auxin transport in roots act redundantly in the establishment to PIN polarization and to the establishment of the apical-basal and maintenance of robust morphogenetic maxima critical for axis in young embryos further implicates tissue-specific auxin root meristem activity. In contrast, local auxin biosynthesis is production in the control of plant development (Robert et al., essential (and cannot be fully compensated for by transport) 2013). Thus, in some developmental contexts, the location of for root responses to hormone ethylene, as well as for flower auxin sources is of critical physiological importance, and long- development. Thus, local auxin production and transport in distance transport may not suffice for the generation of morpho- plants represent partially redundant physiological mecha- genetic auxin gradients in all tissues. On the other hand, a uniform nisms that work together to confer greater robustness and supply of IAA in the growth medium can partially suppress the tunability to instructional auxin gradients, enabling develop- root meristem defects of strong auxin biosynthetic mutants (Ste- mental plasticity and adaptation of plants to changing environ- panova et al., 2011), suggesting that a non-localized source of mental conditions. auxin readily available to roots may be all that is needed for the transport machinery to generate the auxin maxima required for RESULTS AND DISCUSSION root development. Consistent with the latter argument are the conclusions of the first experimental and mathematical modeling Auxin Produced in the Root Is Required for Maintaining approaches that showed how the auxin transport system alone Functional Root Meristems could generate robust auxin gradients in roots with any (i.e., Prior studies (Stepanova et al., 2008) demonstrated that in wei8 non-local) sources of auxin (Grieneisen et al., 2007). Alternative tar2 a deficiency in auxin synthesized via the IPyA pathway models were later built under the assumption that minor produc- results in a loss of robust auxin gradients and leads to root mer- tion and degradation of auxin do take place in every root cell, istem degeneration. Herein, to dissect the role of shoot- versus but higher auxin production rates occur in the quiescent center root-derived auxin in root meristem maintenance, we per- (QC) and columella initials to account for the local synthesis of formed reciprocal grafting of shoots and roots of wild-type auxin in the root meristem (Band et al., 2014). If locally made (WT) and wei8 tar2 seedlings introgressed with an auxin- auxin was removed from these models, the gradient was not responsive reporter DR5:GFP at 3 days post germination, i.e., qualitatively affected (Band et al., 2014), arguing against the prior to mutant seedlings displaying any obvious signs of root crucial role of local sources of auxin.